Long chain perfluoroalkyl iodides are prepared by the conventional telomerization of tetrafluoroethylene (TFE) with pentafluoroethyl iodide, which can be thought of, formally, as the insertion of TFE into the C—I bond. This telomerization yields perfluoroalkyl iodides with a chain length distribution of varying TFE insertions. Most current processes produce chain lengths from 8 carbons to 20 carbons in length. Perfluoroalkyl iodides have many uses as a key starting material in the preparation of surface modifying products, such as repellants, stain and soil resist agents, as well as surfactants. The current trend in these end use markets is to exclude the 8-carbon and longer chain lengths, and focus on shorter chain perfluoroalkyl iodides as starting materials. This work specifically targets the production of straight chain perfluorobutyl iodide and perfluorohexyl iodide (C4-I and C6-I perfluoroalkyl iodides, respectively) from higher order (longer chain) perfluoroalkyl iodides, such as perfluoro-octyl iodide (C8-I) or mixtures of C8-I and higher homologue perfluoroalkyl iodides.
Bertocchio et al., in U.S. Pat. No. 5,268,516, disclose a process for preparation of shorter chain perfluoroalkyl iodides in a thermal telomerization of TFE with pentafluoroethyl iodide or heptafluoroisopropyl iodide by adjusting feed concentration and location of the TFE feed. This process is distinct from that reported herein in that it is a telomerization of TFE.
Becker et al., in German Patent Application DE 4,410,551A1, disclose a process for preparing short chain perfluoroalkyl iodides by reaction of longer chain perfluoroalkyl iodides (greater than 8 carbons in the perfluoroalkyl) with shorter chain perfluoroalkyl iodides (6 carbons or less in the perfluoroalkyl) to produce the desired short chain perfluoroalkyl iodides as well as inert perfluoroalkanes as a by-product. Becker et al., added iodine in the process to increase selectivity of the perfluoroalkyl iodide over the perfluoroalkanes, although this also decreased the overall conversion of reactants to the desired products. The addition of iodine (I2) increases the presence of water into the system, potentially producing a second inert byproduct, 1-hydroperfluoroalkanes.
The use of iodine causes several issues in processes. Iodine introduces opportunities for line plugs, a need for iodine recycling and neutralization, and the potential hydrogen iodide formation. As observed by Becker, iodine may also lead to production of unwanted inert by-products, such as 1-hydroperfluoroalkanes.
U.S. Pat. No. 8,258,354 B2 (to Jacobson et al.) discloses a process to produce F(CF2CF2)2—I and F(CF2CF2)3—I from F(CF2CF2)m—I and F(CF2CF2)p—I, where m is an integer greater than or equal to 3, and p is an integer equal to or lower than 2, and which process is operative without the use of added iodine.
Despite these disclosures, the yields of F(CF2CF2)2—I and F(CF2CF2)3—I are borderline in terms of being high enough to enable a commercially viable process. There is a need for a process for the production of shorter chain perfluoroalkyl iodides, having high selectivity for F(CF2CF2)2—I and F(CF2CF2)3—I (for example, a combined selectivity for F(CF2CF2)2—I and F(CF2CF2)3—I greater than 45% in the absence of iodine), lower selectivity for inert perfluoroalkanes and operable either in the presence or absence of iodine as a reactant. The present invention meets these needs.